Method for the preparation of cathode active material and method for the preparation of non-aqueous electrolyte

ABSTRACT

A LiFePO 4  carbon composite material is to be synthesized in a single phase satisfactorily to achieve superior cell characteristics. In preparing a cathode active material, starting materials for synthesis of a compound represented by the general formula Li x FePO 4 , where 0&lt;x≦1, are mixed, milled and a carbon material is added to the resulting mass at an optional time point in the course of mixing, milling and sintering. Li 3 PO 4 , Fe 3 (PO 4 ) 2  or its hydrates Fe 3 (PO 4 ) 2 ·nH 2 O, where n denotes the number of hydrates, are used as the starting materials for synthesis of Li x FePO 4 . The the temperature of a product from said sintering is set to 305° C. or less when said product from said sintering is exposed to atmosphere. The oxygen concentration in a sintering atmosphere is set to 1012 ppm in volume or less at the time point of sintering.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method for the preparation of acathode active material, capable of reversibly doping/undoping lithium,and to a method for the preparation of a non-aqueous electrolyte cellemploying ths cathode active material.

[0003] 2. Description of Related Art

[0004] Nowadays, in keeping up with the recent marked progress in theelectronic equipment, researches into re-chargeable secondary cells, aspower sources usable conveniently and economically for prolonged time,are underway. Representative of the secondary cells are leadaccumulators, alkali accumulators and non-aqueous electrolyte secondarycells.

[0005] Of the above secondary cells, lithium ion secondary cells, asnon-aqueous electrolyte secondary cells, have such merits as high outputand high energy density. The lithium ion secondary cells are made up ofa cathode and an anode, including active materials capable of reversiblydoping/undoping lithium ions, and a non-aqueous electrolyte.

[0006] As the anode active material, metal lithium, lithium alloys, suchas Li—Al alloys, electrically conductive high molecular materials, suchas polyacetylene or polypyrrole, doped with lithium, inter-layercompounds, having lithium ions captured into crystal lattices, or carbonmaterials, are routinely used. As the electrolytic solutions, thesolutions obtained on dissolving lithium salts in non-protonic organicsolvents, are used.

[0007] As the cathode active materials, metal oxides or sulfides, orpolymers, such as TiS₂, MoS₂, NbSe₂ or V₂O₅, are used. The dischargingreaction of the non-aqueous electrolyte secondary cells, employing thesematerials, proceeds as lithium ions are eluated into the electrolyticsolution in the anode, whilst lithium ions are intercalated into thespace between the layers of the cathode active material. In charging, areaction which is the reverse of the above-described reaction proceeds,such that lithium is intercalated in the cathode. That is, the processof charging/discharging occurs repeatedly by the repetition of thereaction in which lithium ions from the anode make an entrance into andexit from the cathode active material.

[0008] As the cathode active materials for the lithium ion secondarycells, LiCoO₂, LiNiO₂ and LiMn₂O₄, for example, having a high energydensity and a high voltage, are currently used. However, these cathodeactive materials containing metallic elements having low Clarke numberin the composition thereof, are expensive, while suffering from supplydifficulties. Moreover, these cathode active materials are relativelyhigh in toxicity and detrimental to environment. For this reason, novelcathode active materials, usable in place of these materials, aresearched.

[0009] On the other hand, it is proposed to use LiFePO₄, having anolivinic structure, as a cathode active material for the lithium ionsecondary cells. LiFePO₄ has a high volumetric density of 3.6 g/cm³ andis able to develop a high potential of 3.4 V, with the theoreticalcapacity being as high as 170 mAh/g. In addition, LiFePO₄ in an initialstate has an electro-chemically undopable Li at a rate of one Li atomper each Fe atom, and hence is a promising material as a cathode activematerial for the lithium ion secondary cell. Moreover, since LiFePO₄includes iron, as an inexpensive material rich in supply as naturalresources, it is lower in cost than LiCoO₂, LiNiO₂ or LiMn₂O₄, mentionedabove, while being more amenable to environment because of lowertoxicity.

[0010] However, LiFePO₄ is low in electronic conduction rate, such that,if this material is used as a cathode active material, the internalresistance in the cell tends to be increased. The result is that thepolarization potential on cell circuit closure is increased due toincreased internal resistance of the cell to decrease the cell capacity.Moreover, since the true density of LiFePO₄ is lower than that of theconventional cathode material, the charging ratio of the active materialcannot be increased sufficiently if LiFePO₄ is used as the cathodeactive material, such that the energy density of the cell cannot beincreased sufficiently.

[0011] So, a proposal has been made to use a composite material of acarbon material and a compound of an olivinic structure having thegeneral formula of Li_(x)FePO₄ where 0<x≦1, referred to below as LiFePO₄carbon composite material, as a cathode active material.

[0012] Meanwhile, as a method for the preparation of the LiFePO₄ carboncomposite material, having the olivinic structure, such a method hasbeen proposed which consists in mixing lithium phosphate (Li₃PO₄) andiron phosphate I (Fe₃(PO₄)₂ or hydrates thereof ((Fe₃(PO₄)₂·nH₂O), wheren denotes the number of hydrates, adding carbon to the resulting mixtureand in sintering the resulting mass at a pre-set temperature, such as600° C. or thereabouts.

[0013] However, Fe in LiFePO₄ is in the bivalent state and is liable tooxidation, so that, at a synthesizing temperature of 600° C. orthereabout, the following reaction of the chemical formula (1):

6LiFePO₄+3/2O₂→2Li₃Fe₂(PO₄)₃+Fe₂O₃  (1)

[0014] occurs in air. As may be seen from this chemical formula, if theabove compound is fired in an ordinary atmosphere, that is in air,impurities, such as trivalent Fe compounds, are produced, such that theLiFePO₄ carbon composite material cannot be synthesized in a singlephase.

SUMMARY OF THE INVENTION

[0015] It is therefore an object of the present invention to provide amethod for the preparation of a cathode active material according towhich the LiFePO₄ carbon composite material can be synthesized in asingle phase satisfactorily to realize satisfactory cellcharacteristics.

[0016] It is another object of the present invention to provide a methodfor the preparation of a non-aqueous electrolyte cell which, through useof the so-produced LiFePO₄ carbon composite material, as the cathodeactive material, is superior in cell characteristics, such as cellcapacity or cyclic characteristics.

[0017] In one aspect, the present invention provides a method for thepreparation of a cathode active material including mixing, milling andsintering materials for synthesis of a compound represented by thegeneral formula Li_(x)FePO₄, where 0<x≦1, and adding a carbon materialto the resulting mass at an optional time point in the course of themixing, milling and sintering, employing Li₃PO₄, Fe₃(PO₄)₂ or hydratesFe₃(PO₄)₂·nH₂O thereof, where n denotes the number of hydrates, as thematerials for synthesis of the Li_(x)FePO₄, and setting the oxygenconcentration in a sintering atmosphere to 1012 ppm in volume or less atthe time point of sintering.

[0018] Since the oxygen concentration in the sintering atmosphere isdefined as described above, oxidation of Fe during sintering can beprevented from occurring and hence the single-phase synthesis of theLiFePO₄ carbon composite material can be achieved satisfactorily even ifthe sintering is performed at a temperature e.g., of 600° C. Meanwhile,the milling herein denotes executing the comminuting and mixingsimultaneously.

[0019] In another aspect, the present invention provides a method forthe preparation of a non-aqueous electrolyte cell including a cathodehaving a cathode active material, an anode having an anode activematerial and a non-aqueous electrolyte, wherein, in preparing thecathode active material, sintering starting materials for synthesis of acompound represented by the general formula Li_(x)FePO₄, where 0<x≦1,are mixed and milled, and wherein a carbon material is added to theresulting mass at an optional time point in the course of the mixing,milling and sintering. Li₃PO₄, Fe₃(PO₄)₂ or hydrates Fe₃(PO₄)₂·nH₂Othereof, where n denotes the number of hydrates, is used as the startingmaterials for synthesis of the Li_(x)FePO₄. The oxygen concentration ina sintering atmosphere is set to 1012 ppm in volume or less at the timepoint of sintering.

[0020] Since the oxygen concentration in the sintering atmosphere isdefined as described above, oxidation of Fe during sintering can beprevented from occurring and hence the single-phase synthesis of the theLiFePO₄ carbon composite material can be achieved satisfactorily even ifthe sintering is performed at a temperature e.g., of 600° C. So, withthe use of this composite material as the cathode active material, anon-aqueous electrolyte cell which is superior in cell characteristicsmay be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a longitudinal cross-sectional view showing anillustrative structure of a non-aqueous electrolyte cell embodying thepresent invention.

[0022]FIG. 2 is a graph showing Raman spectral peaks of a carbonmaterial.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Referring to the drawings, preferred embodiments of the presentinvention will be explained in detail.

[0024] Referring to FIG. 1, a non-aqueous electrolyte cell 1, preparedin accordance with the present invention, includes an anode 2, an anodecan 3, holding the anode 2, a cathode 4, a cathode can 5 holding thecathode 4, a separator 6 interposed between the cathode 4 and the anode2, and an insulating gasket 7. In the anode can 3 and in the cathode can5 is charged a non-aqueous electrolytic solution.

[0025] The anode 2 is formed by e.g., a foil of metal lithium as ananode active material. If a material capable of doping/undoping lithiumis used as the anode active material, the anode 2 is a layer of an anodeactive material formed on an anode current collector, which may, forexample, be a nickel foil.

[0026] As the anode active material, capable of doping/undoping lithium,metal lithium, lithium alloys, lithium-doped electrically conductivehigh molecular materials or layered compounds, such as carbon materialsor metal oxides.

[0027] The binder contained in the anode active material may be anysuitable known resin material, routinely used as the binder of the layerof the anode active material for this sort of the non-aqueouselectrolyte cell.

[0028] The anode can 3 holds the anode 2, while operating as an externalanode of the non-aqueous electrolyte cell 1.

[0029] The cathode 4 is a layer of the cathode active material formed ona cathode current collector, such as an aluminum foil. The cathodeactive material, contained in the cathode 4, is able to reversibly emitor occlude lithium electro-chemically.

[0030] As the cathode active material, a composite material of carbonand a compound of an olivinic structure represented by the generalformula Li_(x)FePO₄, where 0<x<≦1.0, that is the LiFePO₄ carboncomposite material, the detailed manufacturing method for which will beexplained subsequently, is used.

[0031] In the following explanation, it is assumed that LiFePO₄ is usedas Li_(x)FePO₄ and a composite material composed of this compound andcarbon is used as the cathode active material.

[0032] The LiFePO₄ carbon composite material is such a material composedof LiFePO₄ particles on the surfaces of which are attached numerousparticles of the carbon material having the particle size appreciablysmaller than the particle size of the LiFePO₄ particles. Since thecarbon material is electrically conductive, the LiFePO₄ carbon compositematerial, composed of the carbon material and LiFePO₄, is higher inelectronic conductivity than e.g., LiFePO₄. That is, since the LiFePO₄carbon composite material is improved in electronic conductivity due tothe carbon particles attached to the LiFePO₄ particles, the capacityproper to LiFePO₄ can be sufficiently manifested. Thus, by using theLiFePO₄ carbon composite material as the cathode active material, thenon-aqueous electrolyte secondary cell 1 having a high capacity can beachieved.

[0033] The carbon content per unit weight in the LiFePO₄ carboncomposite material is desirably not less than 3 wt %. If the carboncontent per unit weight of the LiFePO₄ carbon composite material is lessthan 3 wt %, the amount of carbon particles attached to LiFePO₄ may beinsufficient so that sufficient favorable effect in improving theelectronic conductivity may not be realized.

[0034] As the carbon material forming the LiFePO₄ carbon compositematerial, such a material is preferably used which has an intensity arearatio of diffracted beams appearing at the number of waves of 1570 to1590 cm⁻¹ to the diffracted beams appearing at the number of waves of1340 to 1360 cm⁻¹ in the Raman spectrum of graphite in the Ramanspectroscopy, or the ratio A(D/G), equal to 0.3 or higher.

[0035] The intensity area ratio A(D/G) is defined as being abackground-free Raman spectral intensity area ratio A(D/G) of a G-peakappearing at the number of waves of 1570 to 1590 cm⁻¹ and a D-peakappearing at the number of waves of 1340 to 1360 cm⁻¹ as measured by theRaman spectroscopic method as shown in FIG. 2. The expression“background-free” denotes the state free from noisy portions.

[0036] Among the numerous peaks of the Raman spectrum of Gr, two peaks,namely a peak termed a G-peak appearing at the number of waves of 1570to 1590 cm⁻¹ and a peak termed a D-peak appearing at the number of wavesof 1340 to 1360 cm⁻¹, as discussed above, may be observed. Of these, theD-peak is not a peak inherent in the G-peak, but is a Raman inactivepeak appearing when the structure is distorted and lowered in symmetry.So, the D-peak is a measure of a distorted structure of Gr. It is knownthat the intensity area ratio A (D/G) of the D- and G-peaks isproportionate to a reciprocal of the crystallite size La along the axisa of Gr.

[0037] As such carbon material, an amorphous carbon material, such asacetylene black, is preferably employed.

[0038] The carbon material having the intensity area ratio A (D/G) notless than 0.3 may be obtained by processing such as comminuting with apulverizing device. A carbon material having an arbitrary ratio A (D/G)may be realized by controlling the pulverizing time duration.

[0039] For example, graphite, as a crystalline carbon material, mayreadily be destroyed in its structure by a powerful pulverizing device,such as a planetary ball mill, and thereby progressively amorphized, sothat the intensity area ratio A (D/G) is concomitantly increased. Thatis, by controlling the driving time duration of a pulverizing device,such a carbon material having a desired A (D/G) value not less than 0.3may readily be produced. Thus, subject to pulverization, a crystallinecarbon material may also be preferably employed as a carbon material.

[0040] The powder density of the LiFePO₄ carbon composite material ispreferably not less than 2.2 gg/cm³. If the material for synthesis ofthe LiFePO₄ carbon composite material is milled to such an extent thatthe powder density is not less than 2.2 g/cm³, the resulting LiFePO₄carbon composite material is comminuted sufficiently to realize anon-aqueous electrolyte secondary cell 1 having a higher charging ratioof the cathode active material and a high capacity. Moreover, since theLiFePO₄ carbon composite material is comminuted to satisfy theaforementioned powder density, its specific surface may be said to beincreased. That is, a sufficient contact area may be maintained betweenLiFePO₄ and the carbon material to improve the electronic conductivity.

[0041] If the powder density of the LiFePO₄ carbon composite material isless than 2.2 g/cm³, the LiFePO₄ carbon composite material is notcompressed sufficiently, so that there is a risk that the packing ratioof the active material cannot be improved at the cathode 4.

[0042] On the other hand, the Bulnauer Emmet Teller (BET) specificsurface area in the LiFePO₄ carbon composite material is preferably notless than 10.3 m²/g. If the BET specific surface area of the LiFePO₄carbon composite material is not less than 10.3 m²/g, the surface areaof LiFePO₄ per unit weight can be sufficiently increased to increase thecontact area between LiFePO₄ and the carbon material to improve theelectronic conductivity of the cathode active material.

[0043] The primary particle size of the LiFePO₄ carbon compositematerial is preferably not larger than 3.1 μm. By the primary particlesize of the LiFePO₄ carbon composite material being not larger than 3.1μm, the surface area of LiFePO₄ per unit area may be sufficientlyincreased to increase the contact area between LiFePO₄ and the carbonmaterial to improve the electronic conductivity of the cathode activematerial.

[0044] The binder contained in the layer of the cathode active materialmay be formed of any suitable known resin material routinely used as thebinder for the layer of the cathode active material for this sort of thenon-aqueous electrolyte cell.

[0045] The cathode can 5 holds the cathode 4 while operating as anexternal cathode of the non-aqueous electrolyte cell 1.

[0046] The separator 6, used for separating the cathode 4 and the anode2 from each other, may be formed of any suitable known resin materialroutinely used as a separator for this sort of the non-aqueouselectrolyte cell. For example, a film of a high molecular material, suchas polypropylene, is used. From the relation between the lithium ionconductivity and the energy density, the separator thickness which is asthin as possible is desirable. Specifically, the separator thicknessdesirably is 50 μm or less.

[0047] The insulating gasket 7 is built in and unified to the anode can3. The role of this insulating gasket 7 is to prevent leakage of thenon-aqueous electrolyte solution charged into the anode can 3 and intothe cathode can 5.

[0048] As the non-aqueous electrolyte solution, such a solution obtainedon dissolving an electrolyte in a non-protonic aqueous solvent is used.

[0049] As the non-aqueous solvent, propylene carbonate, ethylenecarbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone,sulforane, 1,2-dimethoxyethane, 1, 2-diethoxyethane, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, methyl propionate, methyllactate, dimethyl carbonate, diethyl carbonate and dipropyl carbonate,for example, may be used. In view of voltage stability, cycliccarbonates, such as propylene carbonate, ethylene carbonate, butylenecarbonate or vinylene carbonate, and chained carbonates, such asdimethyl carbonate, diethyl carbonate and dipropyl carbonate, arepreferably used. These non-aqueous solvents may be used alone or incombination.

[0050] As the electrolytes dissolved in the non-aqueous solvent, lithiumsalts, such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃ or LiN(CF₃SO₂)₂,may be used. Of these lithium salts, LiPF₆ and LiBF₄ are preferred.

[0051] Although the non-aqueous electrolyte cell, explained above, isthe non-aqueous electrolyte secondary cell 1 employing a non-aqueouselectrolyte solution, the present invention is not limited thereto, butmay be applied to such a cell employing a solid electrolyte as thenon-aqueous electrolyte. The solid electrolyte used may be an inorganicsolid electrolyte or a high molecular solid electrolyte, such as gelelectrolyte, provided that the material used exhibits lithium ionconductivity. The inorganic solid electrolyte may be enumerated bylithium nitride and lithium iodide. The high molecular solid electrolyteis comprised of an electrolyte salt and a high molecular compounddissolving it. The high molecular compound may be an etheric highmolecular material, such as poly(ethylene oxide), cross-linked or not, apoly(methacrylate) ester based compound, or an acrylate-based highmolecular material, either alone or in combination in the state of beingcopolymerized or mixed in the molecules. In this case, the matrix of thegel electrolyte may be a variety of high molecular materials capable ofabsorbing and gelating the non-aqueous electrolyte solution. As thesehigh molecular materials, fluorine-based high molecular materials, suchas, for example, poly(vinylidene fluoride) or poly(vinylidene fluoride—CO—hexafluoropropylene), etheric high molecular materials, such aspolyethylene oxide, cross-linked or not, or poly(acrylonitrile), may beused. Of these, the fluorine-based high molecular materials areparticularly desirable in view of redox stability.

[0052] The method for the preparation of the non-aqueous electrolytecell 1, constructed as described above, is hereinafter explained.

[0053] First, a composite material of Li_(x)FePO₄ and the carbonmaterial, as a cathode active material, is synthesized by amanufacturing method as now explained.

[0054] For synthesizing the cathode active material, Li_(x)FePO₄ as astarting material for synthesis is kneaded together, milled andsintered. At an optional time point in the course of the mixing, millingand sintering, a carbon material is added to the kneaded startingmaterials for synthesis. As the Li_(x)FePO₄ starting materials forsynthesis, Li₃PO₄, Li₃(PO₄)₂ or a hydrate Fe₃(PO₄)₂·nH₂O thereof where ndenotes the number of hydrates, are used.

[0055] In the following, such a case is explained in which lithiumphosphate Li₃PO₄ and a hydrate Fe₃(PO₄)₂·8H₂O thereof, synthesized asexplained below, are used as starting materials for synthesis, and inwhich, after adding a carbon material to these starting materials forsynthesis, a number of process steps are executed to synthesize theLiFePO₄ carbon composite material.

[0056] First, the LiFePO₄ starting materials for synthesis and thecarbon material are mixed together to form a mixture by way of a mixingstep. The mixture from the mixing step is then milled by a millingprocess, and the milled mixture then is fired by way of a sinteringprocess.

[0057] In the mixing process, lithium phosphate and iron phosphate Ioctahydrate are mixed together at a pre-set ratio and further added towith a carbon material to form a mixture.

[0058] This iron phosphate I octahydrate, used as a starting materialfor synthesis, is synthesized by adding disodium hydrogen phosphateduodecahydrate (2Na₂HPO₄·12H₂O) to an aqueous solution obtained ondissolving iron phosphate heptahydrate (FeSO₄·7H₂O) in water and byallowing the resulting mass to dwell for a pre-set time. The reaction ofsynthesis of iron phosphate I octahydrate may be represented by thefollowing chemical formula (2):

3FeSO₄·7H₂O+2Na2HPO4·12H₂O→Fe₃(PO₄)₂·8H₂O+2Na₂SO₄+37H₂O  (2).

[0059] In iron phosphate I octahydrate, as the material for synthesis,there is contained a certain amount of Fe³⁺ from the synthesis process.If Fe³⁺ is left in the material for synthesis, a trivalent Fe compoundis generated by sintering to obstruct single-phase synthesis of theLiFePO₄ carbon composite material. It is therefore necessary to add areducing agent to the starting materials for synthesis prior tosintering and to reduce Fe³⁺ contained in the starting materials forsynthesis at the time of firing to Fe²⁺.

[0060] However, there is a limitation to the capability of the reducingagent in reducing Fe³⁺ to Fe²⁺ by the reducing agent, such that, if thecontent of Fe³⁺ in the starting materials for synthesis is excessive, itmay be an occurrence that Fe³⁺ is not reduced in its entirety but isleft in the LiFePO₄ carbon composite material.

[0061] It is therefore desirable that the content of Fe³⁺ in the totaliron in the iron phosphate I octahydrate be set to 61 wt % or less. Bylimiting the content of Fe³⁺ in the total iron in the iron phosphate Ioctahydrate to 61 wt % or less from the outset, single-phase synthesisof the LiFePO₄ carbon composite material can be satisfactorily achievedwithout allowing Fe³⁺ to be left at the time of firing, that is withoutgenerating impurities ascribable to Fe³⁺.

[0062] It should be noted that, the longer the dwell time in generatingiron phosphate I octahydrate, the larger becomes the content of Fe³⁺ inthe generated product, so that, by controlling the dwell time so as tobe equal to a preset time, iron phosphate I octahydrate having anoptional Fe³⁺ can be produced. The content of Fe³⁺ in the total iron inthe iron phosphate I octahydrate can be measured by the Mesbauer method.

[0063] The carbon material added to the starting materials for synthesisacts as a reducing agent for reducing Fe³⁺ to Fe²⁺, at the time ofsintering, even if Fe²⁺ contained in iron phosphate I octahydrate as thestarting materials for synthesis is oxidized to Fe³⁺ by oxygen inatmosphere or due to sintering. Therefore, even if Fe³⁺ is left in thestarting materials for synthesis, impurities may be prevented from beinggenerated to assure single-phase synthesis of the LiFePO₄ carboncomposite material. Moreover, the carbon material acts as an antioxidantfor preventing oxidation of Fe²⁺ contained in the starting materials forsynthesis to Fe³⁺. That is, the carbon material prevents oxidation toFe³⁺ of Fe²⁺ by oxygen present in atmosphere and in a firing oven priorto or during sintering.

[0064] That is, the carbon material acts not only as an electrificationagent for improving the electronic conductivity of the cathode activematerial but also as a reducing agent and as an antioxidant. Meanwhile,since this carbon material is a component of the LiFePO₄ carboncomposite material, there is no necessity of removing the carbonmaterial following synthesis of the LiFePO₄ carbon composite material.The result is the improved efficiency in the preparation of the LiFePO₄carbon composite material.

[0065] It is noted that the carbon content per unit weight of theLiFePO₄ carbon composite material be not less than 3 wt %. By settingthe carbon content per unit weight of the LiFePO₄ carbon compositematerial to not less than 3 wt %, it is possible to utilize the capacityand cyclic characteristics inherent in LiFePO₄ to its fullest extent.

[0066] In the milling process, the mixture resulting from the mixingprocess is subjected to milling in which pulverization and mixing occursimultaneously. By the milling herein is meant the powerful comminutingand mixing by a ball mill. As the ball mill, a planetary ball mill, ashaker ball mill or a mechano-fusion may selectively be employed.

[0067] By milling the mixture from the mixing process, the startingmaterials for synthesis and the carbon material can be mixedhomogeneously. Moreover, if the starting materials for synthesis iscomminuted by milling, the specific surface area of the startingmaterials for synthesis can be increased, thereby increasing the contactpoints of the starting materials for synthesis to accelerate thesynthesis reaction in the subsequent sintering process.

[0068] It is desirable that, by milling the mixture containing thestarting materials for synthesis, the particle size distribution of theparticle size not less than 3 μm be not larger than 22% in terms of thevolumetric integration frequency. With the particle size distribution ofthe starting materials for synthesis in the above range, the startingmaterials for synthesis has a surface area sufficient to produce surfaceactivity for carrying out the synthesis reaction. Thus, even if thesintering temperature is of a low value of e.g., 600° C. which is lowerthan the melting point of the starting materials for synthesis, thereaction efficiency is optimum, thus realizing the single-phasesynthesis of the LiFePO₄ carbon composite material satisfactorily.

[0069] Moreover, the milling is desirably executed so that the powderdensity of the LiFePO₄ carbon composite material will be 2.2 g/cm³ orhigher. By comminuting the starting materials for synthesis to give theabove defined powder density, the specific surface area of LiFePO₄ andhence the contact area between LiFePO₄ and the carbon material can beincreased to improve the electronic conductivity of the cathode activematerial.

[0070] In the firing process, the milled mixture from the millingprocess is sintered. By sintering the mixture, lithium phosphate can bereacted with iron phosphate I octahydrate to synthesize LiFePO₄.

[0071] The synthesis reaction of LiFePO₄ may be represented by thefollowing reaction formula (3):

Li₃PO₄+Fe₃(PO₄)₂·nH₂O→3LiFePO₄+nH₂O  (3)

[0072] where n denotes the number of hydrates and is equal to 0 for ananhydride. In the chemical formula (3), Li₃PO₄ is reacted with Fe₃(PO₄)₂or its hydrate Fe₃(PO₄)₂·nH₂O where n denotes the number of hydrates.

[0073] As may be seen from the chemical formula (3), no by-product isyielded if Fe₃(PO₄)₂ is used as a starting materials for synthesis. Onthe other hand, if Fe₃(PO₄)₂·nH₂O is used, water, which is non-toxic, isby-produced.

[0074] Heretofore, lithium carbonate, ammonium dihydrogen phosphate andiron acetate II, as syntheses materials, are mixed at a pre-set ratioand sintered to synthesize LiFePO₄ by the reaction shown by the chemicalformula (4):

Li₂CO₃+2Fe(CH₃COO)₂+2NH₄H₂PO₄→2LiFePO₄+CO₂+H₂+2NH₃+4CH₃COOH  (4)

[0075] As may be seen from the reaction formula (4), toxic by-products,such as ammonia or acetic acid, are generated on sintering with theconventional synthesis method for LiFePO₄. So, a large-scale equipment,such as gas collector, is required for processing these toxicby-products, thus raising the cost. In addition, the yield of LiFePO₄ islowered because these by-products are generated in large quantities.

[0076] According to the present invention, in which Li₃PO₄, Fe₃(PO₄)₂ orits hydrate Fe₃(PO₄)₂·nH₂O, where n denotes the number of hydrates, isused as the starting material for synthesis, targeted LiFePO₄ can beproduced without generating toxic by-products. In other words, safety insintering may be appreciably improved as compared to the conventionalmanufacturing method. Moreover, while a large-scale processing equipmentis heretofore required for processing toxic by-products, themanufacturing method of the present invention yields only water, whichis innoxious, as a by-product, thus appreciably simplifying theprocessing step to allow to reduce size of the processing equipment. Theresult is that the production cost can be appreciably lower than ifammonia etc which is by-produced in the conventional system has to beprocessed. Moreover, since the by-product is yielded only in minorquantities, the yield of LiFePO₄ may be improved significantly.

[0077] Although the sintering temperature in sintering the mixture maybe 400 to 900° C. by the above synthesis method, it is preferably 600°C. or thereabouts in consideration of the cell performance. If thesintering temperature is less than 400° C., neither the chemicalreaction not crystallization proceeds sufficiently such that there isthe risk that the phase of impurities such as Li₃PO₄ of the startingmaterials for synthesis may persist and hence the homogeneous LiFePO₄cannot be produced. If conversely the sintering temperature exceeds 900°C., crystallization proceeds excessively so that the LiFePO₄ particlesare coarse in size to decrease the contact area between LiFePO₄ and thecarbon material to render it impossible to achieve sufficientdischarging capacity.

[0078] During sintering, Fe in the LiFePO₄ carbon composite materialsynthesized is in the bivalent state. So, in the temperature of theorder of 600° C. as the synthesis temperature, Fe in the LiFePO₄ carboncomposite material is promptly oxidized to Fe³⁺ by oxygen in thesintering atmosphere in accordance with the chemical formula shown bythe chemical formula (5):

6FiFePO₄+3/2O₂→2Li₃Fe₂(PO₄)₃+Fe₂O₃  (5)

[0079] so that impurities such as trivalent Fe compounds are produced toobstruct the single-phase synthesis of the LiFePO₄ carbon compositematerial.

[0080] So, inert gases, such as nitrogen or argon, or reducing gases,such as hydrogen or carbon monoxide, are used as the sinteringatmosphere, while the oxygen concentration in the sintering atmosphereis prescribed to a range within which Fe in the LiFePO₄ carbon compositematerial is not oxidized, that is to not larger than 1012 ppm in volume.By setting the oxygen concentration in the sintering atmosphere to 1012ppm in volume or less, it is possible to prevent Fe from being oxidizedeven at the synthesis temperature of 600° C. or thereabouts to achievethe single-phase synthesis of the LiFePO₄ carbon composite material.

[0081] If the oxygen concentration in the sintering atmosphere is 1012ppm in volume or higher, the amount of oxygen in the sinteringatmosphere is excessive, such that Fe in the LiFePO₄ carbon compositematerial is oxidized to Fe³⁺ to generate impurities to obstruct thesingle-phase synthesis of the LiFePO₄ carbon composite material.

[0082] As for takeout of the sintered LiFePO₄ carbon composite material,the takeout temperature of the sintered LiFePO₄ carbon compositematerial, that is th temperature of the LiFePO₄ carbon compositematerial when exposed to atmosphere, is desirably 305° C. or lower. Onthe other hand, the takeout temperature of the sintered LiFePO₄ carboncomposite material is more desirably 204° C. or lower. By setting thetakeout temperature of the LiFePO₄ carbon composite material to 305° C.or lower, Fe in the sintered LiFePO₄ carbon composite material isoxidized by oxygen in atmosphere to prevent impurities from beingproduced.

[0083] If the sintered LiFePO₄ carbon composite material is taken out inan insufficiently cooled state, Fe in the LiFePO₄ carbon compositematerial is oxidized by oxygen in atmosphere, such that impurities tendto be produced. However, if the LiFePO₄ carbon composite material iscooled to too low a temperature, the operating efficiency tends to belowered.

[0084] Thus, by setting the takeout temperature of the sintered LiFePO₄carbon composite material to 305° C. or lower, it is possible to preventFe in the sintered LiFePO₄ carbon composite material from being oxidizedby oxygen in atmosphere and hence to prevent impurities from beinggenerated to maintain the operation efficiency as well as to synthesizethe LiFePO₄ carbon composite material having desirable characteristicsas the cell with high efficiency.

[0085] Meanwhile, the cooling of the as-sintered LiFePO₄ carboncomposite material is effected in a sintering furnace. The coolingmethod used may be spontaneous cooling or by forced cooling. However, ifa shorter cooling time, that is a higher operating efficiency, isenvisaged, forced cooling is desirable. In case the forced cooling isused, it is sufficient if a gas mixture of oxygen and inert gases, oronly the inert gases, are supplied into the sintering furnace so thatthe oxygen concentration in the sintering furnace will be not higherthan the aforementioned oxygen concentration, that is 1012 ppm in volumeor less.

[0086] Although the carbon material is added prior to milling, it may beadded after milling or after sintering.

[0087] However, if the carbon material is added after sintering, thereducing effect in sintering or the effect in prohibiting oxidationcannot be realized but the carbon material is used only for improvingthe electrical conductivity. Therefore, in case the carbon material isadded after the sintering, it is necessary to prevent Fe³⁺ from beingleft by other means.

[0088] In the carbon material is added after sintering, the productsynthesized by sintering is not the LiFePO₄ carbon composite materialbut is LiFePO₄. So, after adding the carbon material, synthesized bysintering, milling is again carried out. By again carrying out themilling, the carbon material added is comminuted and more liable to beattached to the surface of LiFePO₄. By the second milling, LiFePO₄ andthe carbon material is mixed together sufficiently so that thecomminuted carbon material can be homogeneously attached to the surfaceof LiFePO₄. Thus, even when the carbon material is added after thesintering, it is possible to obtain a product similar to one obtained incase the addition of the carbon material is effected prior to milling,that is the LiFePO₄ carbon composite material. On the other hand, themeritorious effect similar to that described above can be realized.

[0089] The non-aqueous electrolyte secondary cell 1, employing theLiFePO₄ carbon composite material, obtained as described above, as thecathode active material, may, for example, be prepared as follows:

[0090] As the anode 2, the anode active material and the binder aredispersed in a solvent to prepare a slurried anode mixture. Theso-produced anode mixture is evenly coated on a current collector anddried in situ to form a layer of the anode active material to producethe anode 2. As the binder of the anode mixture, any suitable knownbinder may be used. In addition, any desired known additive may be addedto the anode mixture. It is also possible to use metal lithium, whichbecomes the anode active material, directly as the anode 2.

[0091] As the cathode 4, the LiFePO₄ carbon composite material, as thecathode active material, and the binder, are dispersed in a solvent toprepare a slurried cathode mixture. The so-produced cathode mixture isevenly coated on the current collector and dried in situ to form a layerof the cathode active material to complete the cathode 4. As the binderof the cathode active material, any suitable known binder may be used,whilst any desirable known additive may be added to the cathode mixture.

[0092] The non-aqueous electrolyte may be prepared by dissolving anelectrolyte salt in a non-aqueous solvent.

[0093] The anode 2 is held in the anode can 3, the cathode is held inthe cathode can 5 and the separator 6 formed by a porous polypropylenefilm is arranged between the anode 2 and the cathode 4. The non-aqueouselectrolytic solution is injected into the anode can 3 and into thecathode can 5. The anode can 3 and the cathode can 5 are caulkedtogether and secured with the interposition of the insulating gasket 7in-between to complete a coin-shaped non-aqueous electrolyte cell 1.

[0094] The non-aqueous electrolyte cell 1, prepared as described above,having the LiFePO₄ carbon composite material as the cathode activematerial, has a high charging ratio of the cathode active material andis superior in electronic conductivity. Thus, with this non-aqueouselectrolyte cell 1, lithium ion doping/undoping occurs satisfactorily sothat the cell may be of a larger capacity. In addition, since thesuperior cyclic characteristics inherent in LiFePO₄ may be manifestedsufficiently, the cell may be of a larger capacity and superior incyclic characteristics.

[0095] There is no particular limitation to the shape of the non-aqueouselectrolyte cell 1 of the above-mentioned embodiment, such that the cellmay be cylindrically-shaped, square-shaped, coin-shaped orbutton-shaped, while it may be of a thin type or of a larger format.

EXAMPLES

[0096] The present invention is hereinafter explained based on specifiedtest results. For checking into the meritorious effect of the presentinvention, the LiFePO₄ carbon composite material was synthesized and,using the so-produced LiFePO₄ carbon composite material as the cathodeactive material, a non-aqueous electrolyte cell was prepared to evaluateits characteristics.

Example 1

[0097] A LiFePO₄ carbon composite material was prepared as a cathodeactive material. The method for producing the cathode active material ishereinafter explained.

[0098] First, Li₃PO₄ and Fe₃(PO₄)₂·8H₂O were mixed to give an elementaryratio of lithium to iron equal to 1:1. To the resulting mixture wereadded acetylene black powders, as the amorphous carbon material, so thatthe acetylene black powders account for 10 wt % o the sintered productin its entirety. The mixture and the alumina balls, each 10 mm indiameter, were charged into an alumina pot 100 mm in diameter, with theweight ratio of the mixture to the alumina balls equal to 1:2. Themixture was milled using a planetary ball mill. As this planetary ballmill, a planetary rotating pot mill for test, manufactured by ITOSEISAKUSHO KK under the trade name of LA-PO4, was used, and the mixturewas milled under the following conditions:

[0099] Conditions for Planetary Ball Milling

[0100] radius of rotation about sun gear: 200 mm

[0101] number of revolutions about the sun gear: 250 rpm

[0102] number of revolutions about a planetary gear itself: 250 rpm

[0103] driving time duration: 10 hours.

[0104] The milled mixture was charged into a ceramic crucible andsintered for five hours at a temperature of 600° C. in an electricalfurnace maintained in a nitrogen atmosphere to produce an LiFePO₄ carboncomposite material.

[0105] This sintering was carried out as follows:

[0106] An atmospheric line provided with a gas mixer capable of mixingair and high purity nitrogen and a sintering unit having a zirconiaoxygen densitometer between the atmospheric line and the sinteringfurnace were provided.

[0107] The pre-sintering starting materials, prepared as describedabove, were charged into a ceramic crucible and, as the influx amountsof air and nitrogen were adjusted so that the oxygen concentration inthe sintering furnace of the electrical furnace was equal to 3 ppm involume, the materials charged were sintered for five hours at atemperature of 600° C. to prepare a product.

Example 2

[0108] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 11 ppm in volume.

Example 3

[0109] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 67 ppm in volume.0.

Example 4

[0110] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 184 ppm in volume.

Example 5

[0111] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 290 ppm in volume.0.

Example 6

[0112] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 612 ppm in volume.

Example 7

[0113] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 867 ppm in volume.5.

Example 8

[0114] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 1012 ppm in volume.

Comparative Example 1

[0115] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 1129 ppm in volume.

Comparative Example 2

[0116] A product was prepared in the same way as in Example 1, exceptthat the influx amounts of air and nitrogen were adjusted to give theoxygen concentration in the sintering furnace of the electrical furnaceequal to 1563 ppm in volume.

[0117] X-ray diffractometry was carried out on the products obtained asdescribed above. The results are shown in Table 1 along with the oxygenconcentration in the sintering furnace. In the Table 1, the sampleproducts matched to the powder X-ray diffraction lines stated inJCPDS-No. 401499 and in which no diffraction lines were observed aremarked ∘ as those sample in which single-phase synthesis of the LiFePO₄carbon composite material has occurred, whereas those sample productsnot matched to the powder X-ray diffraction lines stated in JCPDS-No.401499 or those sample products matched to the powder X-ray diffractionlines stated in JCPDS-No. 401499 but in which other diffraction lineswere observed, are marked x. TABLE 1 oxygen concentration (ppm)synthesizability Ex. 1 3 ∘ Ex. 2 11 ∘ Ex. 3 67 ∘ Ex. 4 184 ∘ Ex. 5 290 ∘Ex. 6 612 ∘ Ex. 7 867 ∘ Ex. 8 1012 ∘ Comp. Ex. 1 1129 X Comp. Ex. 2 1563X

[0118] It is seen from Table 1 that, in the synthesis of the LiFePO₄carbon composite material, the Examples 1 to 8, in which the oxygenconcentration of the sintering atmosphere at the time of sintering is1012 ppm in volume or less, are matched to the diffraction lines of thepowder X rays stated in the JCPDS-No. 401499, whilst no otherdiffraction rays are identified, meaning that the single-phase synthesisof the LiFePO₄ carbon composite material has been achieved. This ispresumably ascribable to the fact that, since the oxygen concentrationof the sintering atmosphere is set to a preset range low enough toprohibit the oxidizing reaction of Fe, that is to 1012 ppm in volume orless, the single-phase synthesis of the LiFePO₄ carbon compositematerial has been satisfactorily achieved without producing oxidizingreactions of Fe.

[0119] It is also seen from Table 1 that, in the synthesis of theLiFePO₄ carbon composite material, the Comparative Examples 1 and 2, inwhich the oxygen concentration in the sintering atmosphere at the timeof sintering is 1012 ppm in volume or higher, are not matched to thepowder X-ray diffraction lines stated in JCPDS-NO. 401499 or are matchedto the powder X-ray diffraction lines stated in JCPDS-No. 401499 butother diffraction lines were observed, so that, in these ComparativeExamples 1 and 2, no single-phase synthesis of the LiFePO₄ carboncomposite material has occurred. This is presumably ascribable to thefact that, since the oxygen concentration of the sintering atmosphere istoo high, the Fe oxidizing reaction occurs to produce impurities toimpede the single-phase synthesis of the LiFePO₄ carbon compositematerial.

[0120] It may be said from above that, by setting the oxygenconcentration of the sintering atmosphere at the time of sintering to1012 ppm in volume or less, in the synthesis of the LiFePO₄ carboncomposite material, it is possible to prevent impurities ascribable tothe oxidizing reaction of Fe from being produced even under thecondition of the sintering temperature of 600° C. thus realizing thesingle-phase synthesis of the LiFePO₄ carbon composite material.

[0121] A non-aqueous electrolyte cell, employing the LiFePO₄ carboncomposite material, produced as described above, was prepared.

Example 9

[0122] 95 parts by weight of the LiFePO₄ carbon composite material, asthe cathode active material, prepared in Example 1, and 5 parts byweight of poly (vinylidene fluoride), in the form of fluorine resinpowders, as a binder, were mixed together and molded under pressure toform a pellet-shaped cathode having a diameter of 15.5 mm and athickness of 0.1 mm.

[0123] A foil of metal lithium was then punched to substantially thesame shape as the cathode to form an anode.

[0124] Then, a non-aqueous electrolyte solution was prepared bydissolving LiPF₆ in a solvent mixture comprised of equal volumes ofpropylene carbonate and dimethyl carbonate, at a concentration of 1mol/l, to prepare a non-aqueous electrolyte solution.

[0125] The cathode, thus prepared, was charged into the cathode can,while the anode was held in the anode can and the separator was arrangedbetween the cathode and the anode. The non-aqueous electrolytic solutionis injected into the anode can and into the cathode can. The anode canand the cathode can 5 were caulked and secured together to complete acoin-shaped non-aqueous electrolyte cell 19.

Example 10

[0126] A coin-shaped test cell was prepared in the same way as inExample 9 except using the product obtained as in Example 5 as thecathode active material.

Example 11

[0127] A coin-shaped test cell was prepared in the same way as inExample 9 except using the product obtained as in Example 8 as thecathode active material.

Comparative Example 3

[0128] A coin-shaped test cell was prepared in the same way as inExample 9 except using the product obtained as in Comparative Examples 2as the cathode active material.

[0129] The coin-shaped test cells of the Examples 9 to 11 and theComparative Example 3, prepared as described above, thecharging/discharging tests were conducted as now explained to find theinitial discharging capacity and the capacity upkeep ratio after 50cycles.

[0130] Test of Charging/discharging Cyclic Characteristics

[0131] The charging/discharging cyclic characteristics were evaluatedbased on the volume upkeep ratio after repeated charging/discharging.

[0132] Each test cell was charged at a constant current and, at a timepoint the cell voltage reached 4.2 V, the constant current charging wasswitched to constant voltage charging and charging was carried out asthe cell voltage was kept at 4.2 V. The charging was terminated at atime point the current value fell to 0.01 mA/cm² or less. Each test wasthen discharged. The discharging was terminated at a time point the cellvoltage fell to 2.0 V.

[0133] With the above process as one cycle, 50 cycles were carried out,and the discharging capacity at the first cycle and that at the fifthcycle were found. The ratio of the discharging capacity at the fiftiethcycle (C2) to the discharging capacity at the first cycle (C1)(C2/C1)×100 was found as the capacity upkeep ratio. Meanwhile, both thecharging and the discharging were carried out at ambient temperature(25° C.), as the current density at this time was set to 0.1 mA/cm². Theresults are shown in Table 2. TABLE 2 oxygen initial capacity afterupkeep concentration synthesizable discharging 50 cycles ratio on (ppm)or no capacity (mAh/g) (%) Ex.9   3 ∘ 161 153 95 Ex.10  290 ∘ 160 151 94Ex.11 1012 ∘ 158 134 85 Comp. 1563 x  59  33 56 Ex. 3

[0134] It may be seen from Table 2 that, in the Examples 9 to 11,employing the cathode active material in which single-phase synthesis ofthe LiFePO₄ carbon composite material was achieved, the initialdischarging capacity, the discharging capacity at the fiftieth cycle(C2) and the capacity upkeep ratio were satisfactory, whereas, in theComparative Example 3, employing the cathode active material in whichreliable single-phase synthesis of the LiFePO₄ carbon composite materialwas not achieved, the initial discharging capacity, the dischargingcapacity at the fiftieth cycle (C2) and the capacity upkeep ratio weredecreased appreciably, thus indicating that the product prepared in theComparative Example 3 were not proper as the cathode active material.

[0135] From the foregoing, it may be said that, in order to produce anLiFePO₄ carbon composite material proper as a cathode active material,the oxygen concentration in the sintering atmosphere at the time ofsintering needs to be not larger than 1012 ppm in volume.

[0136] Next, a polymer cell was prepared to evaluate itscharacteristics.

Example 12

[0137] A gelated electrode was prepared as follows: First,polyvinylidene fluoride, in which was copolymerized 6.0 wt % ofhexafluoropropylene, a non-aqueous electrolyte and dimethyl carbonate,were mixed, agitated and dissolved to a sol-like electrolytic solution.To the sol-like electrolytic solution was added 0.5 wt % of vinylenecarbonate VC to form a gelated electrolytic solution. As the non-aqueouselectrolyte solution, such a solution obtained on mixing ethylenecarbonate EC and propylene carbonate PC at a volumetric ratio of 6:4 andon dissolving LiPF₆ at a rate of 0.85 mol/kg in the resulting mixturewas used.

[0138] A cathode was then prepared as follows: First, 95 parts by weightof the LiFePO₄ carbon composite material, prepared in Example 8, and 5parts by weight of poly (vinylidene fluoride), in the form of fluorineresin powders, as a binder, were mixed together, and added to withN-methyl pyrrolidone to give a slurry, which slurry was coated on analuminum foil 20 μm in thickness, dried in situ under heating andpressed to form a cathode coating film. A gelated electrolytic solutionthen was applied to one surface of the cathode coating film and dried insitu to remove the solvent. The resulting product was punched to acircle 15 mm in diameter, depending on the cell diameter, to form acathode electrode.

[0139] The anode then was prepared as follows: First, 10 wt % offluorine resin powders, as a binder, were mixed into graphite powders,and added to with N-methyl pyrrolidone to form a slurry, which then wascoated on a copper foil, dried in situ under heating and pressed to forman anode coating foil. On one surface of the anode coating foil wasapplied a gelated electrolytic solution and dried in situ to remove thesolvent. The resulting product was punched to a circle 16.5 mm indiameter, depending on the cell diameter, to form an anode electrode.

[0140] The cathode, thus prepared, was charged into the cathode can,while the anode was held in the anode can and the separator was arrangedbetween the cathode and the anode. The anode can and the cathode canwere caulked and secured together to complete a coin-shaped lithiumpolymer cell.

[0141] The polymer cell of Example 12, prepared as described above, wasput to the aforementioned test on charging/discharging cycliccharacteristics to find the initial discharging capacity and capacityupkeep ratio after 30 cycles. The results are shown in Table 3. TABLE 3oxygen concentration initial discharging volumetric upkeep ratio (ppm)capacity (mAh/g) after 30 cycles (%) Ex. 12 1012 156 95.8

[0142] As may be seen from Table 3, both the initial dischargingcapacity and capacity upkeep ratio after 30 cycles are of satisfactoryvalues. From this, it may be seen that the cathode active materialprepared in accordance with the manufacturing method of the presentinvention gives meritorious effects, such as improved discharge capacityand improved cyclic characteristics, even in case the gelatedelectrolyte is used in place of the non-aqueous electrolyte as thenon-aqueous electrolytic solution.

What is claimed is:
 1. A method for the preparation of a cathode activematerial comprising: mixing, milling and sintering materials forsynthesis of a compound represented by the general formula Li_(x)FePO₄,where 0<x≦1, and adding a carbon material to the resulting mass at anoptional time point in the course of said mixing, milling and sintering;employing Li₃PO₄, Fe₃(PO₄)₂ or hydrates Fe₃(PO₄)₂·nH₂O thereof, where ndenotes the number of hydrates, as the materials for synthesis of saidLi_(x)FePO₄; and setting the temperature of a product from saidsintering to 305° C. or less when said product from said sintering isexposed to atmosphere. setting the oxygen concentration in a sinteringatmosphere to 1012 ppm in volume or less at the time point of sintering.2. A method for the preparation of a non-aqueous electrolyte cellincluding a cathode having a cathode active material, an anode having ananode active material and a non-aqueous electrolyte, wherein inpreparing said cathode active material, sintering starting materials forsynthesis of a compound represented by the general formula Li_(x)FePO₄,where 0<x≦1, are mixed, milled and a carbon material is added to theresulting mass at an optional time point in the course of said mixing,milling and sintering; Li₃PO₄, Fe₃(PO₄)₂ or hydrates Fe₃(PO₄)₂·nH₂Othereof, where n denotes the number of hydrates, are used as thestarting materials for synthesis of said Li_(x)FePO₄; and thetemperature of a product from said sintering is set to 305° C. or lesswhen said product from said sintering is exposed to atmosphere. theoxygen concentration in a sintering atmosphere is set to 1012 ppm involume or less at the time point of sintering.
 3. The method for thepreparation of a non-aqueous electrolyte cell according to claim 2wherein said non-aqueous electrolyte comprising a non-aqueouselectrolyte including a dissolved electrolyte in a non-aqueous solvent.4. The method for the preparation of a non-aqueous electrolyte cellaccording to claim 2 wherein said non-aqueous electrolyte is a solidelectrolyte.
 5. The method for the preparation of a non-aqueouselectrolyte cell according to claim 2 wherein said anode is a materialcapable of doping/undoping lithium.
 6. The method for the preparation ofa non-aqueous electrolyte cell according to claim 2 wherein said anodeis a carbon material.